1. Field of the Invention
The present invention relates to a magnetic recording medium having a magnetic layer formed by coating a magnetic powder composition on a base film and a preparation thereof.
2. Description of the Prior Art
When an electric resistance of a magnetic layer of a magnetic recording medium is high, electrostatic force is formed to adhere dust to cause a drop-out. Sometimes, electrostatic discharge noise is formed. In order to reduce high electric resistance which causes formation of the electrostatic force, it has been considered to incorporate carbon or a surfactant type antistatic agent in a magnetic layer. However, it causes deterioration of a sensitivity and unstable running.
When steel balls having mohs' hardness of 6 are used in a step of dispersing a magnetic powder in a binder with the steel balls, if an abrasive powder having mohs' hardness of more than 6 is incorporated, the surface of the steel balls is slightly ground by the abrasive powder in the mixing step, whereby the electric conductive steel powder is dispersed in the magnetic powder composition to reduce the electric resistance of the magnetic layer. The incorporation of the abrasive powder is also effective for reinforcing the magnetic layer against wearing of the surface of the magnetic recording tape in high speed recording operation.
The incorporation of the abrasive powder is preferable to reduce an electric resistance or to reinforce a magnetic layer. However there is a problem of wearing of a magnetic head which contacts with the magnetic recording tape when the abrasive powder is incorporated for these effects.
As shown in FIG. 1, the particle size of the abrasive powder highly relates to the electric resistance and the magnetic head wearing. The effect to the electric resistance is opposite to the effect to the magnetic head wearing. If a coarse abrasive powder is incorporated to reduce the electric resistance at a desired degree, the magnetic head wearing is too high and life of the magnetic head is shortened. On the other hand, when the fine abrasive powder is incorporated, it is necessary to incorporate a large amount of the abrasive powder whereby the magnetic characteristics of the magnetic recording tape are inferior.